Recently the interest has grown to harmonics generation and related
multiphoton processes in graphene and other 2D nanostructures with the
similar properties [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]} due to the
achievements of production of new nanomaterials. Many experiments already
exist where the theoretical predictions have been justified, in particular,
the experiment [17]} with generation of 9th harmonic in graphene in
infrared regime, and generation of harmonics extending to the 13th order
in 2D semiconductor [23]} and in solids [24]}. In these
nanostructures at very high carrier mobility, with increasing pump wave
intensity one can achieve an ultrafast excitation regime and it is important
that threshold values of intensities of electromagnetic (EM) radiation for
observation of nonlinear phenomena compared with free electrons and atoms
are essentially reduced. In particular, for the infrared photons one can
obtain nonlinear effects at the \(10^{9}\)  times smaller intensities of an
external EM field [25]}.
